Log message:
- fine grained X11 dependencies for packages which have either USE_IMAKE
or USE_X11BASE set, but don't include mk/x11.buildlink3.mk directly or
via buildlink3.mks
- introduce BUILDLINK_PREFIX.libXpm as alias for BUILDLINK_PREFIX.xpm
in the !modular case
- fix some cases where the check for libX11 couldn't work at all by using
C++ for compilation without including the proper headers
Verified using a full X11_TYPE=xorg bulk build without additional
breakage. Discussed with salo@, wiz@ and send to packages@ for feedback.

Log message:
Mechanically replace all includes of buildlink3.mk of the following
packages with the modular Xorg equivalent. Those are falling back
to the old location by default, so this commmit doesn't change
dependencies.
graphics/xpm ==> x11/libXpm
fonts/Xft2 ==> x11/libXft
x11/Xfixes ==> x11/libXfixes
x11/xcursor ==> x11/libXcursor
x11/Xrender ==> x11/libXrender
x11/Xrandr ==> libXrandr

Log message:
Point MAINTAINER to pkgsrc-users@NetBSD.org in the case where no
developer is officially maintaining the package.
The rationale for changing this from "tech-pkg" to \
"pkgsrc-users" is
that it implies that any user can try to maintain the package (by
submitting patches to the mailing list). Since the folks most likely
to care about the package are the folks that want to use it or are
already using it, this would leverage the energy of users who aren't
developers.